Many wireless devices, such as cellular telephones, communicate with a base station by using spread spectrum techniques. When using this technique, each underlying information bit is “spread” by one or more digital sequences operating at a higher bit rate than the information itself, resulting in a wider spectrum than would normally be required to transmit the underlying information. By carefully selecting these sequences or codes, multiple users can utilize the same communication channel simultaneously with little or no interference. This technique is called Code Division Multiple Access or CDMA. These spreading sequences have a number of purposes, including privacy, orthogonalization and interference mitigation.
In order to communicate between two devices using spread spectrum, the sequences must be synchronized at both ends of the link. Synchronization of the link can be a multi-step process, which takes an extended period of time to complete.
In addition to the typical communication components, a CDMA device also contains a correlator, used for despreading the received signal and one or more spreading sequence or pseudo-random number (PN) generators. It is these generators that are synchronized with the source of the received signal using code search methods and/or information sent over the air.
A power cut is an unpredicted interruption in power to a cellular phone that can be caused by events such as bouncing battery contacts. If the cellular subscriber unit is in a phone call, a power cut would cause the phone call to be interrupted.
Motorola has previously shipped phones capable of recovering a phone call after a power cut, such as the MicroTAC II for the NTACS system. At the beginning of a phone call, slowly changing or static information necessary to support the call is stored in non-volatile memory. For the NTACS system this information may include such items as RF carrier channel number, color codes, SAT codes, etc. When circuitry in the subscriber unit detects a power cut, the microprocessor recovers the information from memory and re-establishes the phone call.
It is desirable to extend the ability to recover a call after a power cut to the IS-95 CDMA system. At the beginning of a CDMA phone call, the subscriber unit will store information about the call in non-volatile memory. This information will typically include for example, such items as RF carrier channel and long code PN offset. Circuitry in the subscriber unit can detect that a power cut has occurred and initiate an attempt to re-establish the phone call. However, the call cannot be re-established until the subscriber set resynchronizes its internal pseudo-random number generators with those of the base station. Unfortunately, this may take several seconds to accomplish. After approximately 5 seconds of inactivity, the base station will terminate the phone call.
Consequently, there exists a need for a method and apparatus for enabling a wireless device to quickly recover synchronization after a loss of power.